Nitropyridine-1-Oxides as Excellent π-Hole Donors: Interplay between σ-Hole (Halogen, Hydrogen, Triel, and Coordination Bonds) and π-Hole Interactions

Abstract

In this manuscript, we use the primary source of geometrical information, i.e., Cambridge Structural Database (CSD), combined with density functional theory (DFT) calculations (PBE0-D3/def2-TZVP level of theory) to demonstrate the relevance of π-hole interactions in para-nitro substituted pyridine-1-oxides. More importantly, we show that the molecular electrostatic potential (MEP) value above and below the π-hole of the nitro group is largely influenced by the participation of the N-oxide group in several interactions like hydrogen-bonding (HB) halogen-bonding (XB), triel bonding (TrB), and finally, coordination-bonding (CB) (N⁺-O^- coordinated to a transition metal). The CSD search discloses that p-nitro-pyridine-1-oxide derivatives have a strong propensity to participate in π-hole interactions via the nitro group and, concurrently, N-oxide group participates in a series of interactions as electron donor. Remarkably, the DFT calculations show from strong to moderate cooperativity effects between π-hole and HB/XB/TrB/CB interactions (σ-bonding). The synergistic effects between π-hole and σ-hole bonding interactions are studied in terms of cooperativity energies, using MEP surface analysis and the Bader's quantum theory of atoms in molecules (QTAIM).

Keywords: CSD analysis; cooperativity; supramolecular chemistry; π-hole interactions; σ-hole interactions.

Figure 1

(a) Molecular Electrostatic Potential (MEP) surface (0.001 a.u.) of 1. (b) X-ray solid state structures of 1 (NTPYRO). Distances in Å.

Scheme 1

p-Nitropyridine-N-oxide 1, σ-bond donors, electron donors and complexes 3–20.

Scheme 2

Three component systems 11–30.

Figure 2

Partial views of the X-ray structures of NPYOCU (a), MUFZUJ (b), XIHCOG (c), and JUDNAX (d). Distances in Å.

Figure 3

Molecular electrostatic potential (MEP) surfaces plotted onto the van der Waals surface (0.001 a.u.) for complexes 2–5 using the same scale. The MEP values are given in kcal mol⁻¹. The asterisk represents the location of the maximum (V_s,max).

Figure 4

PBE0/def2-TZVP optimized π–hole complexes 6 (a), 7 (b), 8 (c), 9 (d) and 10 (e). Distances in Å.

Figure 5

Optimized complexes 16 (left) and 28 (right). Distances in Å. The equations used to measure the cooperativity energies are also indicated.

Figure 6

Optimized complexes 15 (a) and 19 (b). Distances in Å. The distance in parenthesis corresponds to the binary halogen bonded complex 8.

Figure 7

Distribution of bond ring and cage CPs (green, yellow, and blue spheres) and bond paths in ternary complexes 13 (a), 14 (b), 16 (c), 25 (d) and 17 (e). The values of ρ(r) (a.u.) at the bond CPs that characterize the noncovalent interactions in the ternary complexes are given in blue and their corresponding values in the related binary complexes are indicated in red.